Lipids

• What is a Lipid

  
  

  a heterogeneous class of naturally occurring organic compounds classified together on the basis of common solubility properties
• Lipids are insoluble in water, but soluble in aprotic organic solvents including diethyl ether, chloroform, methylene chloride, and acetone
• Lipids are Amphipathic in nature
• Lipids include:

  • Open Chain forms

  • fatty acids, triacylglycerols, sphingolipids, phosphoacylglycerols, glycolipids,

  • lipid-soluble vitamins

  • prostaglandins, leukotrienes, and thromboxanes

  • Cyclic forms

  • cholesterol, steroid hormones, and bile acids
• Fatty Acids


  an unbranched -chain carboxylic acid, most commonly of 12 - 20 carbons, derived from hydrolysis of animal fats, vegetable oils, or phosphodiacylglycerols of biological membranes
• In the shorthand notation for fatty acids, the number of carbons and the number of double bonds in the chain are shown by two numbers, separated by a colon
• Length of fatty acid plays a role in its chemical character
• Fatty Acids usually contain even numbers of carbons (can contain odd, depending on how they are biosynthesized)
• FA that contain C=C, are unsaturated: If contain only C-C bonds, they are saturated
• Saturated Fatty acids

  Saturated Fatty acids

  A) Lauric

  B) Myristic

  C) Palmitic

  D) Stearic

  E) Arachidic

  
• In most unsaturated fatty acids, the cis isomer predominates; the trans isomer is rare
• Unsaturated fatty acids have lower melting points than their saturated counterparts; the greater the degree of unsaturation, the lower the melting point
• Triacylglycerols are found in adipose tissues, most energy dense molecule
• natural soaps are prepared by boiling triglycerides (animal fats or vegetable oils) with NaOH, in a reaction called saponification (Latin, sapo, soap)
• Soaps form water-insoluble salts when used in water containing Ca(II), Mg(II), and Fe(III) ions (hard water)

  • The salt rinses off
• Salts formed by Saponification

  • Base-catalyzed hydrolysis with salts formed
• Soaps


  • Reactions with acids/bases as catalysts
• Phosphoacylglycerols (Phospholipids)


  When one alcohol group of glycerol is esterified by a phosphoric acid rather than by a carboxylic acid, phosphatidic acid produced
• Phosphoacylglycerols (phosphoglycerides) are the second most abundant group of naturally occurring lipids, and they are found in plant and animal membranes
• Waxes


  A complex mixture of esters of long-chain carboxylic acids and alcohols
• Sphingolipids


  Contain sphingosine, a long-chain amino alcohol
• Sphingolipids are found in plants and animals, and abundant in nervous system
• Sphingolipids has structural similarity to phospholipids

  • Ceramide tells cells to undergo apoptosis

  • Sphingosine tells cells to grow, divide and migrate
• sphingomyelins


  the primary alcohol group of sphingosine is esterified to phosphoric acid, which, in turn, is esterified to another amino alcohol.
• Glycolipids


  a compound in which a carbohydrate is bound to an -OH of the lipid
• many glycolipids are derived from ceramides
• Glycolipids with complex carbohydrate moiety that contains more than 3 sugars are known as gangliosides.
• Steroids


  a group of lipids that have fused-ring structure of 3 sixmembered rings, and 1 five membered ring
• Name this steroid

  A) Cholesterol

  
• Name this Steroid

  A) Testosterone

  B) Estradiol

  C) Progesterone

  
• Cholesterol


  The steroid of most interest in discussion of biological membranes
• Every cell has a cell membrane (plasma membrane)
• Eukaryotic cells also have membrane-enclosed organelles (nuclei, mitochondria…etc)
• Biological Membranes


  Molecular basis of membrane structure is in lipid component(s):

  • polar head groups are in contact with the aqueous environment

  • nonpolar tails are buried within the bilayer

  • the major force driving the formation of lipid bilayers is hydrophobic interaction

  • the arrangement of hydrocarbon tails in the interior can be rigid (if rich in saturated fatty acids) or fluid (if rich in unsaturated fatty acids)
• The polar surface of the bilayer contains charged groups
• The hydrophobic tails lie in the interior of the bilayer
• Plant membranes have a higher percentage of unsaturated fatty acids than animal membranes
• The presence of cholesterol is characteristic of animal rather than plant membranes
• Animal membranes are less fluid (more rigid) than plant membranes
• The membranes of prokaryotes, which contain no appreciable amounts of steroids, are the most fluid